I had this happen to my Hotmail account on ActiveSync (not sure if this is what you're using for your Comcast email). It wouldn't automatically retrieve my emails unless I refreshed it myself, and it would NEVER send emails at all, they would sit in the Outbox like yours are doing.
I had to go into Settings --> Applications --> then into both of the following: Email and Data Manager --> then force stop both of them, clear the cache and the data. Then I had to reboot the phone.
Yes, I had to re-enter all of my information into ActiveSync for my Hotmail account. But it didn't work until I did all of this. Trust me, I tried for almost a week to get it up and running. Drove me nuts. Let me know if this helps or PM me if you aren't sure what I'm talking about.